1. Field of the Invention
The present invention generally relates to a piezoelectric thin-film resonator and a filter, and more particularly, to a piezoelectric thin-film resonator that utilizes the conversions between electric signals and bulk acoustic waves caused in a piezoelectric thin film, and a filter that includes such piezoelectric thin-film resonators.
2. Description of the Related Art
As wireless devices such as portable telephone devices have spread rapidly, there is an increasing demand for small, light-weight resonators, and filters formed by combining those resonators. Conventionally, dielectric filters and surface acoustic wave (SAW) filters have been used. In recent years, however, attention is drawn to piezoelectric thin-film resonators that exhibit excellent characteristics especially at high frequencies and can be made small or monolithic in size, and also to filters that are formed with those piezoelectric thin-film resonators.
Piezoelectric thin-film resonators are classified into FBAR (Film Bulk Acoustic Resonator) types and SMR (Solidly Mounted Resonator) types. A FBAR-type piezoelectric thin-film resonator has a film stack structure that consists of a lower electrode, a piezoelectric film, and an upper electrode, and is formed on a substrate. A hollow space or cavity is formed below the lower electrode at a location (a resonant portion) at which the lower electrode and the upper electrode face each other, with the piezoelectric film being interposed between the lower electrode and the upper electrode. The cavity in an FBAR-type piezoelectric thin-film resonator may be a cavity that is formed between the lower electrode and the substrate by performing wet etching on a sacrifice layer formed on the surface of the substrate, or may be a via hole that is formed in the substrate by performing wet etching or dry etching. A SMR-type piezoelectric thin-film resonator has an acoustic multilayer film, instead of a cavity. The acoustic multilayer film is formed by stacking films having high acoustic impedance and films having low acoustic impedance alternately, and has a film thickness of λ/4(λ: the wavelength of acoustic waves).
An FBAR having a via hole is disclosed in “ZnO/SiO2-Diaphragm Composite Resonator on a Silicon Wafer (K. Nakamura, H. Sasaki, and H. Shimizu, Electronics Letters, vol. 17, No. 14, pp. 507-509, July 1981)”. A FBAR having a cavity is disclosed in Japanese Patent Application Publication No. 60-189307. FIG. 1 is a cross-sectional view of an FBAR having a via hole 18. FIG. 2 is a cross-sectional view of an FBAR having a cavity 18. As shown in FIG. 1, a lower electrode 12, a piezoelectric film 14, and an upper electrode 16 are formed in this order on a substrate 10 that has a SiO2 film 11 formed on its surface and is made of silicon. A cavity 18 (a via hole) is formed in the substrate 10 below a portion at which the lower electrode 12 and the upper electrode 16 face each other. As shown in FIG. 2, a SiO2 film 11 is formed as a supporting film on a substrate 10 made of silicon, so that a cavity 18 (a cavity) can be formed. A lower electrode 12, a piezoelectric film 14, and an upper electrode 16 are formed in this order on the SiO2 film 11. The lower electrode 12 and the upper electrode 16 partially face each other, with the piezoelectric film 14 being interposed between the facing portions of the lower electrode 12 and the upper electrode 16.
Here, the lower electrode 12 and the upper electrode 16 may be made of aluminum, (Al), copper (Cu), molybdenum (Mo), tungsten (W), tantalum (Ta), platinum (Pt), ruthenium (Ru), rhodium (Rh), iridium (Ir), chromium (Cr), titanium (Ti), or the like. The lower electrode 12 and the upper electrode 16 may be stack materials formed by combining some of those materials. The piezoelectric film 14 may be made of aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), lead titanate (PbTiO3), or the like. The substrate 10 may be a silicon substrate, a glass substrate, a GaAs substrate, or the like.
When a high-frequency electric signal is applied between the upper electrode and the lower electrode, acoustic waves are generated by an inverse piezoelectric effect in the piezoelectric film interposed between the upper electrode and the lower electrode, or acoustic waves are generated by the deformation caused by a piezoelectric effect. Those acoustic waves are converted into electric signals. Since such acoustic waves are all reflected by the faces of the upper electrode and the lower electrode exposed to the air, longitudinal oscillatory waves having principal displacement in the thickness direction are generated. Resonance is caused at a frequency at which the total film thickness H of the film stack consisting of the lower electrode, the piezoelectric film, and the upper electrode (including the film added onto the upper electrode) is equal to an integral multiple (n times) of ½ of the wavelength λ of the acoustic waves The resonant frequency F is expressed as: F=nV/(2H), where V represents the propagation velocity of acoustic waves determined by the material. In view of this, the resonant frequency F can be controlled by adjusting the total film thickness H of the stack film, and a piezoelectric thin-film resonator with desired frequency characteristics can be obtained.
Japanese Patent Application Publication No. 2005-536908 of the PCT international publication for a patent application discloses a technique by which an insulating film is provided at the same height as the lower electrode on the substrate, and the step portion is removed from the lower electrode, so as to prevent the degradation of the crystallinity of the piezoelectric film due to the step portion of the lower electrode. Japanese Patent Application Publication No. 2002-140075 discloses a technique by which the step portion of the lower electrode is placed directly on the substrate, so as to prevent the degradation of the crystallinity of the piezoelectric film due to the step portion of the lower electrode. Japanese Patent Application Publication No. 2006-254295 discloses a technique by which the corners of the resist pattern to be used for the formation of the lower electrode are rounded, so as to prevent the degradation of the crystallinity of the piezoelectric film due to the step portion of the lower electrode.
An AlN film is often used as the piezoelectric film, so as to achieve desired acoustic velocity, desired temperature characteristics, and desired sharpness of resonance peaks (the Q value). Particularly, the formation of an AlN film having crystallinity oriented toward the c-axis (oriented in a direction perpendicular to the surface of the lower electrode (the (002) direction)) is one of the essential factors to determine the resonance characteristics. However, the formation of an AlN film having high crystallinity oriented toward the c-axis requires a large amount of energy. For example, where a film is formed by MOCVD (Metal Organic Chemical Vapor Deposition), it is necessary to heat the substrate to 1000° C. or higher. Where a film is formed by PECVD (Plasma Enhanced Chemical Vapor Deposition), it is necessary to provide plasma power and heat the substrate to 400° C. or higher. Also, where a film is formed by a sputtering technique, a temperature rise is caused in the substrate by the sputtering of the insulating film. Therefore, an AlN film has high film stress.
The lower electrode has the step portion, and the side wall of the step portion is tapered. The piezoelectric film is formed to cover the step portion of the lower electrode. As a result, the crystallinity of the piezoelectric film becomes lower at the step portion of the lower electrode, and there is the problem of degradation of the resonance characteristics of the piezoelectric thin-film resonator.
In accordance with Japanese Patent Application Publication No. 2005-536908 of the PCT international publication for a patent application, after an insulating film is formed to cover the step portion of the lower electrode, the surfaces of the lower electrode and the insulating film are flattened by polishing the surfaces so as to remove the step portion of the lower electrode. The film thickness of the lower electrode affects the resonant frequency. Therefore, it is difficult to perform the surface polishing without causing unevenness in the film thickness in the wafer plane and between wafers, while the film thickness control performed on the lower electrode by the surface polishing is essential. Also, the deposition of the insulating film and the surface polishing increase the number of manufacturing procedures. As a result, the productivity becomes lower, and the production costs become higher.
The technique disclosed in Japanese Patent Application Publication No. 2002-140075 can cope with a piezoelectric film having high film stress, and improves the mechanical strength and the Q value. By this technique, however, the electromechanical coupling coefficient (k2) becomes lower.